U.S. patent application number 11/980604 was filed with the patent office on 2008-03-20 for method of preserving fuel cell membrane electrode assembly.
Invention is credited to Takashi Nakagawa, Masatoshi Teranishi, Yoichiro Tsuji.
Application Number | 20080069726 11/980604 |
Document ID | / |
Family ID | 34914537 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080069726 |
Kind Code |
A1 |
Nakagawa; Takashi ; et
al. |
March 20, 2008 |
Method of preserving fuel cell membrane electrode assembly
Abstract
A method of preserving a fuel cell membrane electrode assembly
in which catalyst electrodes are stacked on each surface of a
polymer electrolyte is to preserve the fuel cell membrane electrode
assembly in an airtight package that prevents oxygen, moisture and
a function inhibitor from permeating through the package.
Inventors: |
Nakagawa; Takashi;
(Moriguchi-shi, JP) ; Teranishi; Masatoshi;
(Settsu-shi, JP) ; Tsuji; Yoichiro; (Katano-shi,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
2033 K. STREET, NW
SUITE 800
WASHINGTON
DC
20006
US
|
Family ID: |
34914537 |
Appl. No.: |
11/980604 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11071468 |
Mar 4, 2005 |
|
|
|
11980604 |
Oct 31, 2007 |
|
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Current U.S.
Class: |
422/40 |
Current CPC
Class: |
H01M 8/2457 20160201;
H01M 8/241 20130101; H01M 8/2475 20130101; H01M 8/04231 20130101;
Y02E 60/50 20130101 |
Class at
Publication: |
422/040 |
International
Class: |
B01J 19/00 20060101
B01J019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2004 |
JP |
2004-065196 |
Aug 25, 2004 |
JP |
2004-245566 |
Claims
1. A method of preserving a fuel cell membrane electrode assembly
in which a catalyst electrode is arranged on each surface of a
polymer electrolyte in a layered manner, the method comprising:
covering the membrane electrode assembly with a flexible polymeric
film with moisture permeability of 0.1 g/m.sup.2/day or less and
oxygen permeability of 0.1 cc/m.sup.2/day/atm or less; providing a
space around a portion of the polymer electrolyte, which projects
beyond the catalyst electrode; and filling the space with inert
gas.
2. The method of preserving a fuel cell membrane electrode assembly
according to claim 1, wherein a surface of the catalyst electrode,
which is positioned opposite to the electrolyte, is in contact with
the polymeric film.
3. The method of preserving a fuel cell membrane electrode assembly
according to claim 1, wherein the inert gas is fuel gas.
Description
[0001] This is a divisional application of Ser. No. 11/071,468,
filed Mar. 4, 2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method of preserving a
fuel cell membrane electrode assembly in which an air electrode and
a fuel electrode are respectively stacked onto the surfaces of a
polymer electrolyte.
[0004] 2. Description of the Related Art
[0005] Recently, a fuel cell has attracted a great deal of
attention as a clean power-generating resource. A variety of types
of fuel cells are provided, and a polymer-electrolyte-type fuel
cell is among them.
[0006] The polymer-electrolyte-type fuel cell has a so-called fuel
cell stack (tandem cell) in which a plurality of smallest units,
each being called a "unit cell" and generating power, are stacked
in series. By providing the fuel cell stack with a unit for
providing oxygen and fuel or a unit for cooling down the tandem
cell, a desired power (voltage) can be obtained through a reaction
between hydrogen and oxygen in each unit cell.
[0007] The unit cell has a separator for conducting electricity and
separating the assemblies which are adjacent to each other when
unit cells are stacked. It is the fuel cell membrane electrode
assembly that mainly controls the reaction between hydrogen and
oxygen.
[0008] The fuel cell membrane electrode assembly includes an
electrolyte made of a polymer ion-exchange membrane that is similar
to a fluoropolymer ion-exchange membrane that has a sulfonic group.
The fuel cell membrane electrode assembly also includes a cathode
catalyst layer that becomes an air electrode and an anode catalyst
layer that becomes a fuel electrode when placed on each surface of
the electrolyte. For example, metal made of platinum and ruthenium
is used for the anode catalyst layer while platinum is used for the
cathode catalyst layer.
[0009] The fuel cell membrane electrode assembly with the structure
as described above causes a reaction between oxygen and hydrogen as
follows: hydrogen gas provided for the fuel electrode is changed
into hydrogen ion in the anode catalyst layer; and the hydrogen ion
is moved, in a state of hydration, to the oxygen electrode's side
through the electrolyte. The ion then reacts with oxygen and
electron and generates water in the cathode catalyst layer. By
repeating the reaction, the fuel cell membrane electrode assembly
generates power (voltage).
[0010] Such a polymer electrolyte fuel cell is manufactured almost
in sequence from a manufacturing of a unit cell, an assembling of a
fuel cell stack through to a final process of assembling a fuel
cell. Therefore, it is possible to manufacture a fuel cell that has
satisfactory functions.
[0011] Today, an era of mass production of fuel cells is about to
coincide with the spread of fuel cell use. There is a good
possibility for a necessity to preserve, for a long time, without
degrading the function thereof, the parts used for assembling a
fuel cell in order to maintain the desired functions throughout the
process of manufacturing. For example, the following method is
conceivable for preserving a fuel cell stack in an atmosphere that
is purged of air (oxygen): purging with the use of inert gas, or
purging with the use of moisture, so that the air (oxygen)
remaining in a fluid channel which is placed in a separator that
transmits oxygen gas and hydrogen gas can be eliminated (see
reference to Japanese Laid-Open Applications No. 2002-93448 and No.
06-251788). Another preservation method involving using an
oxygen-absorbing substance is suggested as a technique of
preserving a fuel cell stack in an atmosphere that is purged of
oxygen (see reference to Japanese Laid-Open Application No.
2000-289380).
[0012] It is possible to preserve the fuel cell stack for a long
time, using the conventional method. Along with the progress in
general use of fuel cells, however, in some cases, only fuel cell
membrane electrode assemblies are manufactured and transported to a
distant place. In this case, the conventional method is not
effective.
[0013] After diligent research through the years in view of the
conventional techniques, the inventors of the present invention
have comes, to discover a cause of the problem generated in the
preservation of fuel cell membrane electrode assembly.
[0014] According to the research, the cause of the problem turns
out to be the use of alcohol in the process of manufacturing a
catalyst that makes up a membrane electrode assembly. For example,
acetylene black carbon powder that supports platinum-ruthenium
metal particles or platinum particles is used as a catalyst powder,
and a pasty catalyst is manufactured by dispersing this catalyst
powder onto ethyl alcohol that contains perfluoro-carbon sulfone
acid powder. The pasty catalyst is spread over a non-woven fabric
made of carbon. A catalyst layer is formed in this way. A membrane
electrode assembly is manufactured by sandwiching the electrolyte
with two catalyst layers whose surface on which the catalyst is
applied faces toward the electrolyte.
[0015] The alcohol remains, however, on the non-woven fabric even
after the membrane electrode assembly is manufactured. If the
membrane electrode assembly is preserved in such condition, oxide
is generated as a result of the reaction between oxygen in the air
and the alcohol, which affects the catalyst. The obtained
observation is that the catalyst layer itself may be degenerated
due to the long-term preservation of the membrane electrode
assembly.
[0016] Another observation is that, in some cases, a dust such as
an organic compound contained in the air may stick to the membrane
electrode assembly depending on the environmental condition in a
factory or a stock room, and the catalyst may be degenerated if an
unnecessary organic compound adheres to the membrane electrode
assembly for a long time.
[0017] In the case where metallic (transition metal in particular)
particulates reach the electrolyte, the metal particles are ionized
since the electrolyte is strongly acid. When the electrolyte to
which ionized metallic particles adhere is provided to the fuel
cell so that the fuel cell is activated, hydroxyl radical is
generated as a result of the reaction between hydrogen peroxide
generated due to the gas that cross leaks from the electrolyte or
the secondary reactions, and the ionized metal adhering to the
metal particulates. The electrolyte is decomposed by the generated
hydroxyl radical. The observation shows that, after the
decomposition of the electrolyte, the electrolyte increasingly
cross leaks so as to accelerate the decomposition of the
electrolyte resulting in decreases in film pressure of the
electrolyte that are evident to the extent that power cannot be
constantly generated.
[0018] It has also been observed that after the exposure to the
oxygen in the air, each of the catalyst layers rises to a high
voltage that is close to 1V. This accelerates oxidization of a
metallic catalyst such as carrier carbon, platinum and ruthenium in
the catalyst layer. Due to the oxidization, the catalyst layer
loses its function as a catalyst or the catalyst melts out of the
catalyst layer which makes the layer deficient.
[0019] Moreover, it turns out that the change in humidity in the
environment where the fuel cell membrane electrode assembly is
preserved causes damage to the electrolyte or to the catalyst layer
after the repetition of expansion and shrinking of the
electrolyte.
[0020] The inventors also discovered that in the case where the
fuel cell membrane electrode assembly falls into one of the above
cases, the fuel cell made of such fuel cell membrane electrode
assembly can be a cause of degradation in initial characteristic
such as voltage and/or current characteristic or a cause of
degradation in serviceability of the fuel cell over a long
term.
[0021] It has also been found that the expansion and shrinking of
the electrolyte also causes change in size, which renders it
difficult or impossible to build up a unit cell.
[0022] Note that in the case where an oxygen-absorbing substance is
placed in a package that has low oxygen permeability, the problem
of oxidization can be prevented. However, the oxygen-absorbing
substance must be carefully selected because in some cases a
substance that accelerates decomposition of electrolyte may be
emitted from the oxygen-absorbing substance.
SUMMARY OF THE INVENTION
[0023] The present invention is conceived in view of the problems
in the prior art and the above observations made by the inventors.
An object of the present invention is to provide a method of
preserving a fuel cell membrane electrode assembly that can
suppress the degradation in the characteristics of the fuel cell
made of the fuel cell membrane electrode assembly that has been
preserved, even in the case where only the fuel cell membrane
electrode assembly is preserved for a long period of time.
[0024] In order to achieve the above object, a method according to
the present invention of preserving fuel cell membrane electrode
assembly having a catalyst electrode stacked on each surface of a
polymer electrolyte, includes: preserving the fuel cell membrane
electrode assembly in an airtight package that prevents oxygen,
moisture and a function inhibitor from permeating through the
package.
[0025] Thus, it is possible to maintain the atmosphere in the
airtight package after the package is sealed, and prevent the
degradation in the functions of the membrane electrode assembly and
the adhesion of unnecessary substances to the membrane electrode
assembly.
[0026] According to the method, an atmosphere in the airtight
package may have a lower oxygen concentration than air.
[0027] In this way, it is possible to prevent the damage to the
catalyst layers and degradation of the catalyst layers which are
caused by oxidization of the membrane electrode assembly or
oxidization of organic substances that remain in the membrane
electrode assembly.
[0028] According to the method, a concentration of fuel gas in an
atmosphere of the airtight package that has just been sealed may be
higher than a concentration of fuel gas in air.
[0029] This causes a reaction between the fuel gas and the residual
oxygen through the catalyst in the membrane electrode assembly so
that the airtight package is filled with the atmosphere that has
little amount of oxygen.
[0030] According to the method, a deoxidizer may be placed in the
airtight package.
[0031] As a result, the airtight package can be easily filled with
an atmosphere that has little amount of oxygen, and thereby, it is
possible to easily prevent the functions of the membrane electrode
assembly from being degraded with time.
[0032] According to the method, a concentration of inert gas in an
atmosphere of the airtight package may be higher than a
concentration of inert gas in air.
[0033] Thus, a concentration of other gas such as oxygen can be
relatively low, which makes it possible to obtain the same
operational effects as can be obtained with the atmosphere that has
a low oxygen concentration.
[0034] According to the method, the airtight package in which the
fuel membrane electrode assembly is placed may be sealed after an
atmosphere of the airtight package is purged with preservative
gas.
[0035] By applying this method, it is possible to preserve the
membrane electrode assembly in a desired atmosphere, and thereby
prevent the functions of the membrane electrode assembly from being
degraded due to a long-term preservation.
[0036] According to the method, the preservative gas may have a
same degree of humidity as humidity inside the airtight package
which has not yet been purged of oxygen.
[0037] Thus, the change in humidity can be suppressed even at an
earlier time of the preservation of the membrane electrode
assembly, and the change in size of the membrane electrode assembly
at an initial stage can be prevented as well. This prevents the
functions such as initial characteristics and serviceability from
being degraded due to a long-term preservation of the membrane
electrode assembly.
[0038] According to the method, the airtight package in which the
membrane electrode assembly is preserved is sealed after an
atmosphere of the airtight package is filled with preservative
gas.
[0039] With the method described above, it is possible to preserve
the membrane electrode assembly in a desired atmosphere within the
airtight container, and thereby to suppress the function
degradation caused by a long-term preservation.
[0040] According to the method, a degree of humidity in the
preservative gas atmosphere may be as same as a humidity of an
atmosphere around the fuel cell membrane electrode assembly that
has not yet been preserved in the airtight package.
[0041] Thus, the change in humidity can be suppressed even at an
earlier time of the preservation of the membrane electrode
assembly, and the change in size of the membrane electrode assembly
at initial stage can be prevented as well. This prevents the
functions such as initial characteristics and serviceability from
being degraded due to a long-term preservation of the membrane
electrode assembly.
[0042] According to the method, an amount of oxygen permeated
through the airtight package is 0.1 ml/(m.sup.2/day/atm) or below
and moisture permeability is 0.1 g/(m.sup.2/day) or below.
[0043] Thus, it is possible to specify the characteristics of the
airtight container that enable the membrane electrode assembly to
be preserved over a long period of time.
[0044] According to the method, a surface of the catalyst electrode
of the fuel cell membrane electrode assembly may be covered with a
protective film that has high oxygen barrier properties.
[0045] Thus, the contact between the membrane electrode assembly
and the oxygen can be directly interrupted, and moreover, the
degradation of the functions such as initial characteristics and
serviceability caused by the long-term preservation of the membrane
electrode assembly can be prevented. Furthermore, a cushioning
function of the film can prevent deficiencies in the membrane
electrode assembly caused by shocks given from outside or a contact
between the membrane electrode assemblies.
FURTHER INFORMATION ABOUT TECHNICAL BACKGROUND TO THIS
APPLICATION
[0046] The disclosure of Japanese Patent Applications No.
2004-065196 filed on Mar. 9, 2004 and No. 2004-245566 filed on Aug.
25, 2004, including specification, drawings and claims is
incorporated herein by reference in its entirety.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] These and other objects, advantages and features of the
invention will become apparent from the following description
thereof taken in conjunction with the accompanying drawings that
illustrate a specific embodiment of the invention. In the
Drawings:
[0048] FIG. 1 is a schematic diagram showing a conventional method
of preserving a fuel cell stack;
[0049] FIG. 2 is a perspective view showing a condition in which a
membrane electrode assembly is preserved according to embodiments
of the present invention;
[0050] FIG. 3 is a cross-sectional view showing a condition in
which a membrane electrode assembly is preserved according to the
embodiments of the present invention;
[0051] FIG. 4 is a cross-sectional view showing a condition in
which plural membrane electrode assemblies are preserved in a
single airtight package, according to the first embodiment;
[0052] FIG. 5 is a cross-sectional view showing a condition in
which a membrane electrode assembly covered with a protective film
is preserved according to a second embodiment;
[0053] FIG. 6 is a cross-sectional view showing a condition in
which plural membrane electrode assemblies, each being covered with
a protective film, are preserved in a single airtight package,
according to the second embodiment;
[0054] FIG. 7 is a sketch of an apparatus for purging inside the
airtight package with the use of preservative gas, according to the
second embodiment;
[0055] FIG. 8 is a sketch of an apparatus for filling the airtight
package with preservative gas, according to the second
embodiment;
[0056] FIG. 9 is a cross-sectional view showing a condition for
rendering the airtight package to have the atmosphere with low
oxygen concentration, according to the second embodiment; and
[0057] FIG. 10 is a graph showing how cell voltages of different
groups vary with time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0058] The following describes the embodiments of the present
invention with reference to the diagrams.
First Embodiment
[0059] FIG. 2 is a perspective view showing a condition in which a
fuel cell membrane electrode assembly is preserved according to the
embodiments of the present invention.
[0060] FIG. 3 is a cross-sectional view in the case of virtually
cutting a membrane electrode assembly 11 preserved in an airtight
package 21.
[0061] As shown in FIGS. 2 and 3, the membrane electrode assembly
11 is preserved in the airtight package 21 that is a bag made of
resin.
[0062] The membrane electrode assembly 11 is a fuel cell membrane
electrode assembly with a structure in which an anode catalyst
layer 13 and a cathode catalyst layer 14 are stacked respectively
on each surface of a polymer electrolyte 12. Each of the catalyst
layers 13 and 14 has a structure in which carbon mesh supports the
catalyst, and is vulnerable against shocks. As shown in detail in
FIG. 3, a gas diffusion layer 15 is placed on one of the surfaces
of the respective catalyst layers 13 and 14.
[0063] The airtight package 21 is made up of materials with high
sealing properties for preventing oxygen, moisture and function
inhibitors from being transmitted. The airtight package 21 may be a
stiff container with a stable form, however, a bag made of a
flexible polymeric film with low moisture permeability and low
oxygen permeability is more preferable because such bag can be
preserved without taking up much space when it is not used for the
preservation of membrane electrode assembly 11.
[0064] More precisely, the airtight package 21 needs moisture
permeability of 0.1 g/m.sup.2/day or less and oxygen permeability
of 0.1 cc/m.sup.2/day/atm or less. This is because degradation with
time of the membrane electrode assembly 11 becomes serious unless
both of the conditions are satisfied. It is desirable if moisture
permeability is 0.01 g/m.sup.2/day or less and oxygen permeability
is 0.01 cc/m.sup.2/day/atm or less. With the conditions satisfied,
the airtight package 21 can serve for a long-term preservation.
[0065] Material can be selected for the airtight package 21 from
the following: a plastic film or a ceramic evaporated film, of
polyvinyl chloride (PVDC), of ethylene-vinylalcohol (EVOH), of
polyvinyl alcohol (PVA) and of polyamide (PA), a single aluminum
evaporated film, a single aluminum foil laminated film, or a film
made by laminating plural barrier materials, or a film made by
compounding the barrier materials used for a barrier layer and a
polymeric film.
[0066] Representative examples of the polymeric film are the
following: a PVDC coat OPP, a PVA coat OPP, an EVOH co-extruded
OPP, a PVDC coat ONY, a multilayered barrier ONY (MXD, an EVOH
co-extruded), a PVDC coat PET, a PVA coat PET, a PVDC coat
cellophane, an EVOH film, an extensible PVA film, a hybrid barrier
coat film, an alumina evaporated film (an alumina (Al2O3)
evaporated PET, a silica (SiOx) evaporated PET, an alumina
evaporated ONY, an alumina evaporated OPP), an aluminum evaporated
film (aluminum evaporated PET, an aluminum evaporated CPP, an
aluminum evaporated OPP, an aluminum evaporated ONY and an aluminum
evaporated PE), or the like.
[0067] Note that OPP stands for bi-oriented propylene, ONY stands
for biaxially-oriented nylon, and CPP signifies inextensible
propylene while MXD denotes polyamide resin with barrier properties
and PET denotes poly-ethylene terephthalate.
[0068] The thickness of the airtight package 21 is not strictly
specified, but it only requires the above materials and to be thick
enough to retain moisture permeability of 0.1 g/m.sup.2/day or less
and oxygen permeability of 0.1 cc/m.sup.2/day/atm or less. Vapor
depositing aluminum onto the film made of the above materials can
surprisingly decrease moisture permeability as well as oxygen
permeability, 20 which makes the airtight package 21 thinner.
[0069] By using the airtight package 21 made of the above-mentioned
material so as to retain moisture permeability of 0.1 g/m.sup.2/day
or less, it is possible to prevent the vapor from leaking out of
the airtight package 21, and thus, to avoid change in amount of
moisture inside the airtight package 21. However, when rapid change
in temperature is generated outside the airtight package 21,
dropwise condensation is generated and humidity changes greatly in
the airtight package 21. It is therefore advisable that the
membrane electrode assembly 11 be preserved in a place, e.g., a
hygrostat and temperature-controlled bath where temperature can be
maintained within a predetermined range, but not in a place where
temperature greatly fluctuates. In this way, it is possible to
prevent the change in the amount of moisture in the polymer
electrolyte as well as the degradation of the polymer
electrolyte.
[0070] The use of the airtight package 21 can also prevent an
inclusion of function inhibitors from outside as well as decrease
in functions of the fuel cell caused by the function inhibitor
adhering to the membrane electrode assembly 11.
[0071] Here, function inhibitors are, for instance, a metallic
element such as an organic substance, iron and transition metal,
and metallic fine particles. Such function inhibitors include not
only a substance that degrades the functions of the membrane
electrode assembly 11 during a long-term preservation, but also a
substance that decreases the function as a fuel cell by adhering to
the membrane electrode assembly 11.
[0072] Note that in preserving the membrane electrode assembly 11
without the catalyst layers 13 and 14 (i.e. at the stage where the
assembly 11 includes only the polymer electrolyte 12 under the
process of assembly), it is possible to prevent the contact between
the polymer electrolyte 12 and oxygen or function inhibitors so as
to maintain the amount of moisture, by preserving the polymer
electrolyte 12 covered with a protective film 16 in the airtight
package 21. It is thus possible to prevent the decrease of
functions of polymer electrolyte 12 even in the case of preserving
the polymer electrolyte 12 for a long period of time.
[0073] FIG. 4 is a cross-sectional view showing the case of
preserving plural fuel cell membrane electrode assemblies 11 in
such manner that they are stacked.
[0074] In the airtight package 21, the plural membrane electrode
assemblies 11 are sealed in such manner that they are stacked
whereas gas diffusion layers 15 are stacked on the catalyst layers
13 and 14 such that a gas diffusion layer 15 of one membrane
electrode assembly 11 is in contact with a gas diffusion layer 15
of another membrane electrode assembly 11.
[0075] Thus, it is also possible to preserve plural membrane
electrode assemblies 11 all together in the airtight package
21.
[0076] By preserving the membrane electrode assembly 11 as
described above, it is possible to preserve a single fuel cell
membrane electrode assembly 11 in the airtight package 21 which
prevents oxygen, moisture and function inhibitor from being
transmitted under the condition where the atmosphere is purged of
oxygen and where change in the amount of moisture (humidity) in the
airtight package 21 is suppressed. This therefore leads to the
prevention of decrease in power-generating function during the
preservation of the fuel cell membrane electrode assembly 11, which
does not cause degradation in the function of the membrane
electrode assembly 11 over a long period.
[0077] It is also possible to fill the airtight package 21 with
water. The oxygen can be eliminated out of the airtight package 21
by pouring water into the airtight package 21. As the membrane
electrode assembly 11 is surrounded by water, change in amount of
moisture in the polymer electrolyte 12 can be prevented. It is
desirable to use purified or distilled water for the filling
because it does not contain any function inhibitors.
Second Embodiment
[0078] The following describes the second embodiment with reference
to the diagrams.
[0079] FIG. 5 is a cross-sectional view showing the condition in
which the membrane electrode assembly 11, whose surface is covered
with the protective film 16, is preserved.
[0080] The membrane electrode assembly 11 and the airtight package
21 are the same as those described in the first embodiment,
therefore, the description is not repeated here.
[0081] The protective film 16 plays a role of directly preventing
the contact between the membrane electrode assembly 11 and oxygen
or function inhibitors that remain in the airtight package 21. The
protective film 16 also serves as a cushioning medium to prevent
the damage to the surface of the catalyst. A polymer film resin
with high barrier properties against oxygen is used as a material
for the protective film 16. Note that the same material as used for
the airtight package 21 may be used for the protective film 16.
[0082] The membrane electrode assembly 11 has a structure in which
the anode catalyst layer 13 and the cathode catalyst layer 14 are
stacked on each surface of the polymer electrolyte 12. The gas
diffusion layer 15 is placed on each of the catalyst layers 13 and
14.
[0083] Even in the case where the membrane electrode assembly 11
has the gas diffusion layers 15, the contact between the surface of
the catalyst and oxygen or function inhibitors is further reduced,
and change in the amount of moisture on the surface of the catalyst
further decreases as well, by covering the surface with the
protective film 16. The covering has a purpose to allow the gas
diffusion layers to serve as protective films, which further
prevents the degradation of the power-generating function of the
membrane electrode assembly 11 due to the long-term preservation.
Therefore, it is desirable to preserve the membrane electrode
assembly 11 that includes the gas diffusion layer 15.
[0084] FIG. 6 is a cross-sectional view showing the case of
preserving plural membrane electrode assemblies 11 in a single
airtight package 21.
[0085] In the airtight package 21, plural membrane electrode
assemblies 11 are sealed in such manner that they are stacked on
each other. By covering the surface of each of the membrane
electrode assemblies 11 with the protective film 16, it is possible
to preserve them in a single airtight package 21 so that the
membrane electrode assemblies 11 contact each other without being
damaged.
[0086] Note that in the present embodiment, each membrane electrode
assembly 11 has a gas diffusion layer 15, however, in some cases,
the membrane electrode assembly 11 without the gas diffusion layer
15 may be preserved.
[0087] Method 1 for creating an atmosphere of the airtight package
21 The following describes a method for creating the atmosphere
that has low oxygen concentration in the airtight package 21.
[0088] The membrane electrode assembly 11 is inserted in a bag-type
airtight package 21 that is party opened.
[0089] As shown in FIG. 7, an air release pipe 31 that exhausts gas
from the airtight package 21 and a supply pipe 32 that provides
preservative gas are inserted, while an opening of the airtight
package 21, in which the membrane electrode assembly 11 is
inserted, is held down.
[0090] The air release pipe 31 is connected to a gas exhaust
apparatus 33, and opens or closes with an air release valve 34
while the supply pipe 32 is connected to a gas supply apparatus 35,
and opens or closes with a supply valve 36.
[0091] Then, the air release valve 34 is released so that the air
in the airtight package 21 is introduced to outside and the
airtight package 21 is vacuumed. After this, the air release valve
34 and the supply valve 36 are switched to be closed, and the
airtight package 21is filled with preservative gas.
[0092] Lastly, the opening is heat-shielded at the same time when
the pipes 31 and 32 are pulled out so that the airtight package 21
is sealed off.
[0093] Note that pipes to be used for the air release pipe 31 and
the supply pipe 32 may be same or different. In the case of using
the same pipe, a valve for switching a line for gas release to a
line for gas supply is used.
[0094] The preservative gas only requires gas with low oxygen
concentration, and it is preferable that the gas has inert gas as a
main component. The gas may contain fuel gas, but still has inert
gas as a main component.
[0095] The fuel gas may be represented by hydrogen gas, and is
provided to the anode's side in the fuel cell.
[0096] In the case where the preservative gas includes such fuel
gas, the fuel gas included in the preservative gas and a slight
amount of oxygen gas that remains in the airtight package 21 or
that comes into the airtight package 21 from outside are used for a
burning reaction generated in the catalyst layer in the membrane
electrode assembly 11. Therefore, neither the oxygen gas remains in
the airtight package 21, nor the catalyst layer is maintained at
high level voltage due to the oxidization of the membrane electrode
assembly 11 or oxygen. This prevents degradation of the functions
of the membrane electrode assembly 11.
[0097] The inert gas includes nitrogen, but may include helium and
argon instead.
[0098] Furthermore, the airtight package 21 may be filled with
inert gas that includes fuel gas based on the following methods:
encapsulating fuel gas after purging the atmosphere with inert gas;
and purging the atmosphere with inert gas after encapsulating fuel
gas. Here, the respective gas can be encapsulated into the airtight
package 21 by separately operating a pipe for encapsulating inert
gas and a pipe for encapsulating fuel gas, by switching between the
valves.
[0099] By allowing the preservative gas to have the same degree of
humidity as humidity of the condition in which the membrane
electrode assembly 11 is manufactured, it is possible to prevent
change in the degree of humidity at the time of gas purging.
[0100] It is described that the opening is sealed based on the
method of thermo compression bonding, however, a method based on
zipping or a method based on compression may be applied instead.
Note that the method based on thermo compression bonding is
desirable for its excellent sealing properties and easy
process.
[0101] In this way, the atmosphere inside the airtight package 21
can be purged with preservative gas, which can fill the airtight
package 21 with a desirable atmosphere.
[0102] Method 2 for creating an atmosphere in the airtight package
21 The following describes another method of filling the airtight
package 21 with the atmosphere that has low oxygen
condensation.
[0103] First, the membrane electrode assembly 11 is inserted into
the bag-type airtight package 21 a part of which is opened.
[0104] Then, the airtight package 21, in which the membrane
electrode assembly 11 is inserted, is placed in a large chamber 51
as shown in FIG. 8.
[0105] The fuel cell membrane electrode assembly 11 covered with
the protective film 16 is inserted into the airtight package 21.
The membrane electrode assembly 11 may include the diffusion layer
15 or may be at a stage where only the catalyst layers 13 and 14
are stacked onto the polymer electrolyte 12.
[0106] The airtight package 21 may be made of metal or resin that
prevents moisture and function inhibitors from entering from
outside.
[0107] The chamber 51 includes the air release pipe 31 for
deaerating the gas within the chamber 51, and the supply pipe 32
for providing preservative gas such as inert gas and fuel gas. The
on-off valves 34 and 36 are respectively connected to the air
release pipe 31 and the supply pipe 32 so that gas release and gas
supply can be arbitrarily performed. The air release pipe 31 and
the supply pipe 32 may be the same pipe. In this case, the
connection to the air release pipe 31 and the supply pipe 32 may be
changed using the valves.
[0108] Then, the on-off valves 34 and 36 are controlled so as to
provide the chamber 51 with inert gas that contains fuel gas, after
the chamber 51 is vacuumed by vacuuming the air. The pressure after
the provision of inert gas is assumed to be 1 atm.
[0109] Lastly, the opening of the airtight package 21 is sealed
within the chamber 51 using the thermo compression bonding method
or the like, so that the air is vacuumed.
[0110] This method is preferable since plural membrane electrode
assemblies 11 can be simultaneously sealed in the chamber 51 filled
with the preservative gas atmosphere according to the capacity of
the chamber 5 1. Even in the case where air is vacuumed out of the
airtight package 21, it is possible to prevent the damages to the
catalyst layers 13 and 14 caused by the atmospheric pressure,
because the atmospheric pressure does not directly affect the
membrane electrode assembly 11.
[0111] Method 3 for creating an atmosphere in the airtight package
21 FIG. 9 is a cross-sectional view showing the condition in which
the membrane electrode assembly 11 is preserved in the airtight
package 21 in which a deoxidizer 3 is placed.
[0112] In the case of using the deoxidizer 3, the deoxidizer 3 is
put into the airtight package 21 so that the airtight package 21 is
sealed off in the air.
[0113] Thus, with the use of deoxidizer 3, it is easy to fill the
airtight package 21 with the atmosphere that has low oxygen
condensation.
[0114] In the case of using the deoxidizer 3, it is also possible
to selectively exclude the oxygen that leaks from outside. Even in
the case where the airtight package 21 has a low degree of closure,
the atmosphere in the airtight package 21 can be continuously
purged of oxygen for a predetermined period of time.
[0115] It is desirable to use an auto-reactive, organic deoxidizer.
Such deoxidizer does not require moisture from outside, therefore,
can prevent change in amount of moisture within the airtight
package 21. In contrast, an iron deoxidizer rapidly absorbs oxygen
and is economic in terms of cost, however, has deficiencies in
depending on moisture and causing change in amount of moisture
within the airtight package 21. It is therefore desirable not to
use iron for fuel cells since it becomes a function inhibitor that
degrades the power-generating function equipped in the fuel
cell.
EXAMPLE
[0116] The following describes, with reference to FIG. 10, the
result of examining a possibility to prevent the decrease in the
power-generating function as well as the decrease of the
serviceability which are equipped in the membrane electrode
assembly 11 that is preserved based on the preservation method
according to the present invention.
[0117] Firstly, the number of membrane electrode assemblies 11 as
many as they can form at least three fuel batteries was provided. A
group A covered each of the membrane electrode assemblies 11 with
an EVOH film and then left, for a year, the assemblies 11 in the
airtight package 21 filled with an inert gas atmosphere. A group B
left, for a year, the membrane electrode assemblies 11 in the
airtight package 21 filled with the inert gas atmosphere. A group C
wrapped each of the membrane electrode assemblies 11 with a
polyethylene sheet and left the assemblies 11 for a year. After
that, each of the membrane electrode assemblies 11 classified by
each group is stacked together with a separator so as to make a
fuel cell.
[0118] In the examination, hydrogen gas is provided as a fuel for
an anode electrode while air is provided for a cathode electrode as
a gas that contains an oxidizer. The conditions are as follows: the
temperature in a cell is 70.quadrature.; fuel utilization is 70%;
oxygen utilization is 40%; and power flux density is 0.2
A/cm.sup.2.
[0119] FIG. 10 shows how cell voltages of different groups vary
with time until the operating time reaches 3000 hours. Note that a
broken line in the diagram indicates, for comparison, the result of
generating power in a fuel cell that has just been
manufactured.
[0120] According to the observation, at initial stage, cell voltage
of the membrane electrode assemblies 11 (A) and (B) that are
preserved based on the preservation method according to the present
invention is higher than that of the membrane electrode assembly 11
(C) that is preserved based on the conventional preservation
method, and is almost as same as the cell voltage in the case of
generating power in the membrane electrode assembly 11 that has
just been manufactured. With regard to the voltage decay rate until
the operation time reaches 3000 hours, the voltage decay rate of
the cell voltage based on the method according to the present
invention is lower than that of the cell voltage based on the
conventional preservation method, and is almost as same as the
voltage decay rate of the cell voltage in the case of generating
power in the membrane electrode assembly 11 that has just been
manufactured. In the case of further continuing, for a long time,
the power generation in the cell, for which the membrane electrode
assembly 11 that is preserved based on the conventional
preservation method is used, compared to the case of generating
power in the cell for which the membrane electrode assembly 11 that
has just been manufactured is used, the following results are
obtained: the degradation of polymer electrolyte progresses faster
than usual; and the voltage rapidly decreases in a very short time,
which causes inability to generate power. It is also verified that
the cell voltage can be always maintained at higher level in the
group A which covered the membrane electrode assembly 11 with an
EVOH film than the group B which didn't.
[0121] Therefore, the preservation method according to the present
invention can prevent decrease in terms of both power-generating
function and serviceability of the membrane electrode assembly 11
caused by long-term preservation better than the conventional
preservation method. The same effects are obtained in the case
where the membrane electrode assembly 11 is left under the
environmental condition that ordinary temperature is between 20 to
30 degrees Celsius, high temperature is between 50 to 60 degrees
Celsius, and low temperature is between 0 to 10 degrees
Celsius.
[0122] Although only some exemplary embodiments of this invention
have been described in detail above, those skilled in the art will
readily appreciate that many modifications are possible in the
exemplary embodiments without materially departing from the novel
teachings and advantages of this invention. Accordingly, all such
modifications are intended to be included within the scope of this
invention.
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